Feline hyperthyroidism is a common disease of middle-aged to older cats. Higher than normal levels of thyroid hormone production by the thyroid glands cause skin disease in approximately 30% of affected cats. The most commonly seen skin changes include excessive shedding, matted hair coats, excessive grooming, hair loss, abnormal nail growth, greasy coat or dry scaling and thin skin. These changes can be confused with many other skin diseases. Microscopic examination of skin samples obtained by biopsy is often performed in cases of unresolved skin disease. To date, there have been no studies or reports describing the microscopic changes associated with the skin lesions of feline hyperthyroidism. This study will document the microscopic changes detected in the skin of hyperthyroid cats with skin lesions. The information gained will be used to familiarize veterinarians and pathologists with the microscopic changes seen with hyperthyroidism.